Pontiac Firebird Trans Am SD-455

The last of the fast cars comes standard with the sort of acceleration that hasn't been seen in years.

May 1973 By CAR AND DRIVER Photos By HUMPHREY SUTTON

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From the May 1973 issue

Just when we had fast cars relegated to the museum sections, Pontiac has surprised everyone and opened a whole new exhibit. How it ever got past the preview audience in GM's board room is a mystery, but here it is—the car that couldn't happen. And the list of reasons why it couldn't is both long and well known. Compression ratios have plummeted like the Dow Jones in 1929, while factory-sealed lean carburetors are the norm, along with soft ignition and valve timing. It's all summed up by the equation of the year in Detroit: EGR (Exhaust Gas Recirculation) to fight NOx (Nitrous Oxide), at the expense of bhp and mpg. Until now, the realization that exhaust emissions will be Ivory pure in the very near future was hardly an encouraging sign.

The conclusion was inescapable: truly fast street cars would be gone with no chance for an amnesty. But Pontiac has defied all that. Nineteen hundred and seventy-three, unobtrusively perhaps, is the year of the fastest Firebird ever. And it's certified by Uncle Sam to be free from any bad manners. The SD-455 passes the same emission tests after a 4000-mile break-in as does every other production engine. If this were an all-new engine, it would then have to complete a full 50,000-mile certification program. However, since the Federal government recognizes the SD-455 as part of the 455 engine family, the SD version doesn't have to run the durability tests, but can use a deterioration factor defined by a standard 455 four-barrel engine. It is only that kinship to a volume engine that makes it a reality.

And just how fast is this 1973 "Last of the Fast Cars" from Pontiac? One does not jump into that consideration without a bit of meditation to insulate the mind against time-inspired overload. First of all, you must temporarily purge your thought waves of the small displacement engines that have occupied center stage since the neurosurgeons came down on Super Cars. Let your concentration wander over the truly fast cars of today, the 454 Corvettes, 911 Porsches, Panteras and Jaguar V-12s—but only briefly, because they are merely mid-14-second quarter-mile cars.

You must reach a plateau of concentration where the machinery is even swifter. Go all the way back to 1968. Think of the Street Hemis, the L88s, the Boss 429s. When that era of unconcerned euphoria is locked in, sit back and get yourself comfortable to savor this progression of "performance" as determined in 1320 feet. Relish it as you would a Cuban cigar. And no fair looking ahead to the specifications page. We've checked with our lawyers, and they say it's okay to tell you . . . if you can keep a secret.

On the seventh of February, 1973, our test Firebird Trans Am SD-455 swept through the quarter-mile at Orange County International Raceway in Irvine, California in an elapsed time of 13.751 seconds and at a terminal speed of 103.56 mph. That is fast. That, in fact, is outrageously fast even within a five-year-old frame of reference. And it was done in a street legal car—a 1973 street legal car—with a full tank of gas (3854 lb. curb weight), street tires and, wait for it . . . automatic transmission.

There was no cheering throng in the shut-off area, not a single pat on the hoodscoop for the over-achiever 1973 Trans Am. There should have been. The event was easily on a par with Don Garlits' recent five-second runs in a fuel-burning dragster. It proves that once again, blindingly fast acceleration can be yours right off the assembly line.

The secret password is SD-455. Simply check that option code on the order blank, and your Firebird will bristle with under-hood muscle. That is a rare quality these days, especially in a car which has the one-dimensional appearance of a Super Car, but which less ostentatiously has incorporated the subtle improvements that have traditionally sold high performance sport cars.

It is almost a car of bigger than contemporary life dimensions. Among Sporty Cars, only the AMC Javelin and the Firebird offer engines over 351 cu. in. Since all Pontiac engines must be considered a "big" block, the 455 is not new or unusual in the Firebird chassis. But the SD-455 is a whole new breed of car, not just an engine. And you can credit guys like Special Projects Engineer, Herb Adams (sneaky insight: take a look at who has built the Pontiacs occasionally entered in Trans-Am or NAS-CAR Grand National competition the last couple of years), as well as Senior Project Leaders, Skip McCully and Tom Nell. They understand . . . in the face of adversity. The naysayers seemed to be everywhere. The front line of resistance consisted of the bean-counters who swore on a stack of Venturas that Pontiac would lose millions with a production SD-455.

They were supported by a handful of emissions engineers who predicted that certification of a performance engine to 1973 standards would be impossible. As a final reserve, there were the antediluvian members of management . . . men who still scratch their heads over the '64 GTO. Opposition came, was strong, pervasive, almost overwhelming; but Adams, McCully and Nell, in their naive dedication, have won out. They'd much rather think about winning in NASCAR or NHRA than merely chasing hydrocarbons. But if those are to be the rules of the game, they will still play . . but with their own entry. So, in the end, enthusiasts get a stay of execution, and the engineers again have a thoroughbred engine to fret over—all 100 percent legal in government eyes.

The outside of their Super Duty engine is cleverly disguised to look exactly like a standard Firebird 455 V-8. That means a massive lump of cast iron, with its water pump the only piece of aluminum in sight. But the source of strength lies not far below its expanse of turquoise paint. To begin with, there is a strong base of operations. The block is reinforced with thicker bulkheads, 4-bolt main bearings and more material in the camshaft and lifter area. The connecting rods and pistons are forged, and the crankshaft is an iron casting. (A forged crank would be stronger, but Pontiac bypassed the expense with special treatments for the casting. Outside surfaces of the crank are nitrided, and a pressure rolling operation is applied to the journal fillets. That mashes down surface fibers to eliminate minute tensions that often grow into cracks.) Pontiac tests have shown the crank to be one stout piece of iron. Our performance tests show no reason to contradict that claim in any way.

For extra insurance, the engine is assembled with generous clearances. Also, an 80 psi oil system is on hand to keep the lubricant flowing while the engine pumps out all its horsepower. And that power flows off the crankshaft in astounding quantities, an impressive total of 310 net horsepower. The secret to that power, if it is a secret, is that the engine can inhale all the air-fuel mixture it needs. That is really the only avenue to performance these days, since the SD-455, like all those "performance" engines, must squeeze its power out of 91-octane fuel (which limits the compression ratio to a low 8.4 to one) intake and exhaust systems have made up for the low compression ratio. Cylinder head design is unique to this engine, with intake ports of the "constant area" design. Normally, intake passages must wend their way to a combustion chamber, twisting past pushrods, head bolts, water jackets and other incumbent hardware. Avoidance is only part of the problem. In addition, enough material must be left over for sufficient wall thickness in case the casting cores shift—and they do in production.

To accept all these production demands, ports are invariably necked down in certain areas. You wouldn't notice the extra restriction in a low speed engine, such as Pontiac's 350 two-barrel, but it is a hangman's noose to something like the high-revving SD-455, an engine that cannot tolerate emphysema. So Pontiac engineers have discovered another way to do it. Instead of relying on cast iron to seal the intake passage from the pushrod holes, a steel tube does the job. That means material thickness can be thinned down from a minimum of 0.180 in. with cast iron, to 0.030 in. with the tube. That near microscopic difference is the difference. With it, the intake port can shoot through the head like an arrow without bending out of the way to leave extra material around the pushrods.

Other unseen differences include such limited-production features as swirl-polished valves, with the exhausts being larger than standard 455 versions by 0.110 inch. And to match the big intake ports, there is a cast iron intake manifold unique to the SD-455. Initially, aluminum versions were tried, but they posed an insurmountable problem in cold start driveability. At the other end of the head are cast iron "header-type" exhaust manifolds (unique to the SD engine) with round passages to mate with the cylinder head ports. Steel tubing headers would be less restrictive, but they are much less durable . . . not to mention the boisterous exhaust note that would be transmitted to the interior of the car thanks to thin-wall tubing.

And we do mean boisterous. The camshaft is out of the old Ram Air III package, and leaves the intake and exhaust valves hanging open at the same time for 87 degrees of camshaft rotation, so the engine is rough and twitchy at low speeds. But the overlap is necessary for power at the high end, and we found the engine strong right up to its 5700 rpm redline, which is our idea of proper com rpm). Not coincidentally, that happens to be exactly where the Turbo HydraMatic shifts automatically. So quarter-mile runs are largely a matter of a sensitive touch off the line and a heavy foot as soon as the tires lock up. Just leave the shifter in D, and the Pontiac Trans Am SD-455 will rush down the strip run after run with sub-14 second ETs . . without even breathing hard in the process.

That is, unless you leave the ram air scoop as it comes from the factory—with its entrance blocked by a sheet metal panel. Then the stylish mail-slot through to the carburetor is "nonfunctional," and the engine must draw its intake air from under the hood. That warmer and more restrictive air supply is enough to slow the Trans Am by 0.2 seconds and three mph in the quarter-mile. But, as insidious as that little plate may seem, it is there for a reason. Without the scoop and block-off panel, the Trans Am will not pass California drive-by noise standards. The extra plenum volume offered by the big enclosed scoop is enough to muffle the sound of rushing intake air. So every SD-455 rolls off the assembly line with a blocked hood scoop as standard equipment. If you feel your engine deserves cold intake air, removing the plate is a simple task of drilling out three pop rivets.

In the process, however, you will also usher more engine noise into the interior. At idle, the open scoop adds 2.0 dBA; on acceleration, 3.0 dBA. And the mufflers don't help at all. They sound as if they were made for a Honda rather than for the most powerful Pontiac ever. The exhaust barks into the atmosphere through 2.50-in. exhaust pipes and 2.25-in. tail pipes just as it left the engine—with lots of energy. In fact, there is such a commotion behind the rear wheels you're sure the engine rests in the back seat. The din in the interior peaks at 84.0 dBA during acceleration. That is an annoyance by sedan standards, but much quieter than the 92.0 dBA of a 454 Corvette coupe. Yet even with the noise, the interior is quite a pleasant place to be. And it could be even better with some modifications to the massive bucket seats. They are comfortable for the driver, but selfishly consume vast quantities of interior volume. Even though the buckets don't recline or offer much in the way of lateral support, the back rests are ultra thick—so rear seat passengers are shortchanged on leg room.

On the other hand, front seat passengers have all the room they need. And at the same time they are surrounded by strictly first rate materials. The sprawling panels of hard plastic favored in Barracudas and Mustangs are conspicuously absent from the Firebird. Instead, the surfaces you touch are softly padded and covered with vinyl or a smooth-skinned foam. All with a texture as soft and supple as a pigskin glove. Still, that luxurious interior has been a part of every Firebird since 1970. The big improvement in comfort for 1973 comes from a new tire option. They, are, quite naturally, radials—the current shining star of tire construction in Detroit. In fact, they are the same steel-belted Firestone GR70-15 tires specified for the Corvette and Monte Carlo S for 1973. And if you order them on the Trans Am you can throw away your kidney belt. You won't need it over rough surfaces like you do with the standard bias-belted 60-series tires. There is one other important trade-off between the two tire choices.

Pontiac engineers admit that the radial is well below the bias-belted tire in ultimate cornering power, but the radial is much better suited for wet driving. Therefore, we would recommend the radials on ride and safety considerations, but it turns out that they also offer the same or better performance as their fatter counterparts in their effect on steering response. And that is a good thing, because the Trans Am's second most endearing quality—next to its ability to out-accelerate anything not assisted by a rocket motor—is handling. The Firebird's exceptional handling will not lead pipe your senses like the locomotive engine under the hood. Instead, its pleasures are subtle and enjoyable in speed ranges that don't generate tickets. You feel the precision when you roll out of your driveway and down a winding lane at 15 mph. And even as speeds increase, while you turn the flat, "racer-style" steering wheel, your hands turn the car. There are no balky joints, mushy valves or flexible links to confuse the issue. The car tracks exactly as you direct it. And the feedback link between you and the tires' contact patches is as sensitive as your fingertips.

As you step up the speed, the car will corner with barely detectable roll angles. That control is the result of sway bars the size of baseball bats. And they help determine the attitude at the limit as well. With the Firebird's heavy front weight bias (58.4 per cent), understeer, obviously, takes over in the end. But its limits are high enough that it's doubtful you'll ever be bothered by that fact. One aspect of the Trans Am's performance that is potentially bothersome is braking. With the SD-455, the Trans Am will rush up to escape velocities in nearly no time at all. That ability should be matched by equally powerful brakes.

Such was not the case in our test car. The long stopping distances we measured from 70 (226 ft., 0.72G), were largely due to a power booster with very numb reflexes and excessive travel before the pedal can initiate sufficient boost to stop the car. To complicate the problem, the brake pads were not completely bedded in, and therefore, not up to their maximum effectiveness. The result was poor performance coupled with fade apparent after only one panic stop from 70 mph. That tendency should diminish with mileage, but even with hard use, it could take thousands of street miles to properly condition the pads. That is dumb, but Pontiac does offer an alternative . . . for a price. You can buy semi-metallic front brake pads from a special heavy duty Police Package for only $52.85. Just convey the following number to your Pontiac parts man: 5470872. But be prepared for the eternal metallic brake features: better fade resistance, noise during braking and higher pedal efforts when the pads are cold.

All of which you can live with . . . and quite happily if the concept of the Trans Am—a concept that has all but disappeared—is appealing. The Firebird Trans Am is a genus of an automotive species approaching extinction. It could well be the last of its kind. Just the car you need to carry you through the upcoming years of automotive sterility and "zero emissions." Sporty Cars are particularly well cast for that role because they have traditionally offered a rare combination of virtues for the 360-degree enthusiast. They are pleasant to look at—the Firebird is as eye-catching today as it was three and a half years ago at introduction time. And with the Trans Am's plumage of spoilers and air dams, there is no doubt that you're looking at an exceptional piece of equipment. The handling is built in to match engine performance—and without throwing comfort out the window. And if you're brave enough to add the SD-455 option, you won't need a J.C. Whitney catalogue for horsepower. But ultimately, the best compliment bestowed on the Firebird is the fact that, even in its own time, it is becoming a collector's item. . . an exception worth keeping.